I. Field
The present invention relates to satellite based wireless communication systems, in particular, methods for decoding overhead channels transmitted on a satellite link, such as the reverse link (up-link) to a geosynchronous (GEO) satellite.
II. Background
The reverse link channels for wireless communications typically consist of the pilot channel, traffic/data channel, and the overhead channels. The pilot channel provides a reference signal known to the receiver at the gateway demodulator and is used for channel estimation. The traffic channel conveys the data packets from the user application. The overhead channels are used to indicate to the gateway the information necessary to operate the forward link and reverse link.
A user terminal communicating with a gateway through a GEO satellite may need to support multiple data rates on the forward and reverse links. The link from the gateway on the ground to the user terminal via the satellite is the “forward link” and the link from the user terminal to the gateway on the ground via the satellite is the “reverse link”. To indicate the data rate that is supportable on the forward link, the terminal uses a Data Rate Control (DRC) channel transmitted on the reverse link. Similarly, to indicate the data rate at which the reverse link data channel is transmitted, the terminal uses the reverse rate indicator (RRI) channel. The RRI channel is transmitted at all times, even when no data packets are being sent on the reverse traffic channel. This may be accomplished by indicating a “zero data rate” or “null rate” on the reverse traffic channel. The DRC and RRI channels are examples of overhead channels. They can be carried in two separate overhead channels or alternately, can be combined into one overhead channel. An overhead channel may be separated from the other channels by means of orthogonal code multiplexing, such as with Walsh codes. For example, each overhead channel may be encoded, modulated and spread with its own unique Walsh code to maintain channel separation from the pilot channel, the traffic channel, and the other overhead channels.